Multi-antenna techniques may significantly increase the data rates and reliability of a wireless communication system. Performance of the system is in particular improved if both the transmitter and the receiver are equipped with multiple antennas. This use of multiple antennas results in a multiple-input multiple-output (MIMO) communication channel and such systems and/or related techniques are commonly referred to as MIMO.
Evolved UTRAN (E-UTRAN), also called for LTE, is a standard that is currently under development. A core component in LTE is the support of MIMO antenna deployments and MIMO related techniques. In particular, for the downlink a spatial multiplexing mode with channel dependent precoding is supported. The spatial multiplexing mode is aimed for high data rates in favorable channel conditions. In this mode, an information carrying symbol vector sk is on the base station (eNodeB in LTE) side multiplied by an NT×r precoder matrix denoted asWNT×r.
The matrix is often chosen to match the characteristics of the NR×NT MIMO channel, where NR and NT represents the number of receive and transmit antennas, respectively. The r symbols in sk each correspond to a layer and r is referred to as the transmission rank. LTE uses OFDM and hence the NR×1 vector received by the user equipment (UE) for a certain resource element on subcarrier k (or alternatively data resource element number k), assuming no inter-cell interference, is thus modeled byyk=HWNT×rsk+ek 
where ek is a noise vector obtained as realizations of a random process.
The UE may, based on channel measurements in the forward link, transmit recommendations to the base station of a suitable precoder to use. A single precoder that is supposed to cover a large bandwidth (wideband precoding) may be fed back. It may also be beneficial to match the frequency variations of the channel and instead feed back a frequency-selective precoding report, e.g. several precoders, one per subband.
In the field of high rate multi-antenna transmission, one of the most important characteristics of the channel conditions is the so-called channel rank. Roughly speaking, the channel rank may vary from one up to the minimum number of transmit and receive antennas and characterizes how many layers the channel can support for a transmission. In conjunction with precoding, adapting the transmission to the channel rank involves using as many layers as the channel rank. This is facilitated by feedback information from the receiver to the transmitter, Such feedback information may comprise not only which precoder or precoders to use but also a recommendation of the transmission rank (possibly implicitly as part of the precoder information) and quality assessments of the layers/codewords. The latter is often referred to as CQI, Channel Quality Indication while the feedback information related to transmission rank may be referred to as rank indication (RI) which may be used in conjunction with precoder matrix indicator(s) (PMIs) to unambiguously point out one or more precoder matrices.
The payload size of the feedback information may be particularly large if frequency-selective precoding is used. Several precoders/PMIs may then need to be signaled and this may lead to a large signaling overhead. In order to avoid such a large signaling overhead also for the forward link signaling (e.g. in the downlink from eNodeB to UE), it is possible for the transmitter to exploit the fact that the receiver knows what it recommended and hence, instead of explicitly signaling one or more of the recommended precoders, confirm to the receiver that the data transmission is using the same precoders and transmission rank as the receiver recommended. This is often referred to as precoder confirmation/verification and is part of the control information associated with a data transmission in the forward link.
In practice, the feed back reports are far from ideal due to time-variations of the channel and feedback delay, bit errors in the feedback link and mismatch between the assumptions on certain parameters the receiver use for computing/selecting feedback information and what the actual parameter values at the transmitter are. The scheduling bandwidth is one important example of such a parameter.
In LTE, the User Equipment, UE, reports a single recommended rank to the base station (eNodeB in LTE) obtained by inspecting the channel quality as seen over the maximum possible scheduling bandwidth (which may have been semi-statically configured to be smaller than the system bandwidth). The actual bandwidth used when the UE is scheduled might however be considerably smaller. In scenarios with a frequency-selective channel, this means that there is a great risk that the effective rank on the scheduled bandwidth might be entirely different from the “average” transmission rank recommended by the UE.
Documents LG Electronics 3GPP Draft; R1-074194 Downlink Control Signaling for SU-MIMO_LGE, 20071003 3rd Generation Partnership Project (3GPP), Mobile Competence Centre; 650, route des Lucioles; F-06921 Sophia-Antipolis Cedex; France RAN WG1, Shanghai, China; 20071003 R1-074194 “Downlink Control Signaling for SU-MIMO_LGE” XP050107723 and LG electronics 3GPP Draft; R1-074200, 20071002 3rd Generation Partnership Project (3GPP), Mobile Competence Centre; 650, route des Lucioles; F-06921 Sophia-Antipolis Cedex; France, RAN WG1, Shanghai, China; 20071002 R1-074200 “On the implementation of rank override using codeword DTX” XP050107729 disclose rank override.